The Greater Rhea (Rhea americana) is also known as the Grey, Common or American Rhea.
The native range of this flightless bird is the eastern part of South America; it is not only the largest species of the genus Rhea but also the largest American bird alive.
t is also notable for its reproductive habits, and for the fact that a group has established itself in Germany in recent years.
In its native range, it is known as ñandú (Spanish) or ema (Portuguese).
The adults have an average weight of 20–27 kg (44–60 lb) and 129 cm (51 in) long from beak to tail; they usually stand about 1.50 m (5 ft) tall. The males are generally bigger than the females, males can weigh up to 40 kilograms (88 lb) and measure over 150 cm (59 in) long.
The legs are long and strong and have three toes. The wings of the American Rhea are rather long; the birds use them during running to maintain balance during tight turns.
Greater Rheas have a fluffy, tattered-looking plumage. The feathers are gray or brown, with high individual variation, In general, males are darker than females. Even in the wild – particularly in Argentina – leucistic individuals (with white body plumage and blue eyes) as well as albinos occur. Hatchling Greater Rheas are grey with dark lengthwise stripes.
There are five subspecies of the Greater Rhea, which are difficult to distinguish and whose validity is somewhat unclear; their ranges meet around the Tropic of Capricorn:
- R. americana americana – campos of northern and eastern Brazil
- R. americana intermedia – Uruguay and extreme southeastern Brazil (Rio Grande do Sul province)
- R. americana nobilis – eastern Paraguay, east of Rio Paraguay
- R. americana araneipes – chaco of Paraguay and Bolivia and the Mato Grosso province of Brazil
- R. americana albescens – plains of Argentina south to the Rio Negro province.
The main subspecific differences are the extent of the black coloring of the throat and the height. However, rheas differ so little across their range that without knowledge of the place of origin, it is essentially impossible to identify captive birds to subspecies.
Distribution, ecology and status
The Greater Rhea is endemic to Argentina, Bolivia, Brazil, Paraguay and Uruguay. This species inhabits grassland dominated e.g. by satintail (Imperata) and bahiagrass (Paspalum) species, as well as savanna, scrub forest, chaparral, and even desert and palustrine lands, though it prefers areas with at least some tall vegetation. It is absent from the humid tropical forests of the Mata Atlantica and Planalto uplands along the coast of Brazil and extends south to 40° latitude. During the breeding season (spring and summer), it stays near water.
A small population of the Greater Rhea has become established in Germany. Three pairs escaped from a farm in Groß Grönau, Schleswig-Holstein, in August 2000. These birds survived the winter and succeeded in breeding in a habitat similar to that of their South American one. They eventually crossed the Wakenitz River and settled in Mecklenburg-Vorpommern in the area around and particularly to the north of Thandorf village. As of the late 2000s, the population is estimated to be 7 birds, and in 2001 18 birds. In October 2008 the population was estimated by two German scientists at around 100 birds (Korthals, A. and F. Philipp).
These rheas are legally protected in Germany in a similar way to native species. In its new home, the Greater Rhea is considered generally beneficial as its browsing helps maintain the habitat diversity of the sparsely-populated grasslands bordering the Schaalsee biosphere reserve.
Food and predators
The bulk of its food consists of broad-leaved dicot foilage and other plantstuffs, particularly seed, and fruit when in season. Favorite food plants include native and introduced species from all sorts of dicot families, such as Amaranthaceae, Asteraceae, Bignoniaceae, Brassicaceae, Fabaceae, Lamiaceae, Myrtaceae, or Solanaceae. Magnoliidae fruit, for example, Duguetia furfuracea (Annonaceae) or avocados (Persea americana, Lauraceae) can be seasonally important. They do not usually eat cereal grains, or monocots in general. However, the leaves of particular grass species like Brachiaria brizantha can be eaten in large quantities, and Liliaceae (e.g. the sarsaparilla Smilax brasiliensis) have also been recorded as foodplants. Even tough and spiny vegetable matter like tubers or thistles is eaten with relish. Like many birds that feed on tough plant matter, the Greater Rhea swallows pebbles which help grind down the food for easy digestion. It is much attracted to sparkling objects and sometimes accidentally swallows metallic or glossy objects.
In fields and plantations of plants they do not like to eat – e.g. cereals or Eucalyptus – the Greater Rhea can be a species quite beneficial to farmers. It will eat any large invertebrate it can catch; its food includes locusts and grasshoppers, true bugs, cockroaches and other pest insects. Juveniles eat more animal matter than adults. In mixed cerrado and agricultural land in Minas Gerais (Brazil), R. a. americana was noted to be particularly fond of beetles. It is not clear whether this applies to the species in general but for example in the pampas habitat, beetle consumption is probably lower simply due to availability while Orthoptera might be more important. The Greater Rhea is able to eat Hymenoptera in quantity. These insects contain among them many that can give painful stings, though the birds do not seem to mind. It may be that this species has elevated resistance to poison, as it readily eats scorpions. But even small vertebrates like rodents, snakes, lizards and small birds are eaten. Sometimes, Greater Rheas will gather at carrion to feed on flies; they are also known to eat dead or dying fish in the dry season, but as vertebrate prey in general not in large quantities.
The natural predators of adult Greater Rheas are limited to the Cougar (Puma concolor) and the Jaguar (Panthera onca). Feral dogs are known to kill younger birds, and the Southern Caracara (Caracara plancus) is suspected to prey on hatchlings. Armadillos sometimes feed on Greater Rhea eggs; nests have been found that had been undermined by a Six-banded Armadillo (Euphractus sexcinctus) or a Big Hairy Armadillo (Chaetophractus villosus) and the rhea eggs were broken apart.
Captive-bred Greater Rheas exhibit significant ecological naïvete. This fearlessness renders them highly vulnerable to predators if the birds are released into the wild in reintroduction projects. Classical conditioning of Greater Rhea juveniles against predator models can prevent this to some degree, but the personality type of the birds – whether they are bold or shy – influences the success of such training. In 2006, a protocol was established for training Greater Rheas to avoid would-be predators, and for identifying the most cautious animals for release.
Greater Rheas breed in the warmer months, between August and January depending on the climate. Males are simultaneously polygynous, females are serially polyandrous. In practice, this means that the females move around during the breeding season, mating with a male and depositing their eggs with the male before leaving him and mating with another male.
Males on the other hand are sedentary, attending the nests and taking care of incubation and the hatchlings all on their own.
Recent evidence suggests that dominant males may enlist a subordinate male to roost for them while they start a second nest with a second harem.
The nests are thus collectively used by several females and can contain as many as 80 eggs laid by a dozen females; each individual female’s clutch numbers some 5-10 eggs.
However, the average clutch size is 26 with 7 different female eggs.
Rhea eggs measure about 130 mm × 90 mm (5.1 in × 3.5 in) and weigh 600 g (21 oz) on average; they are thus less than half the size of an ostrich egg. Their shell is greenish-yellow when fresh but soon fades to dull cream when exposed to light.
The nest is a simple shallow and wide scrape in a hidden location; males will drag sticks, grass, and leaves in the area surrounding the nest so it resembles a firebrak as wide as their neck can reach.
The incubation period is 29-43 days. All the eggs hatch within 36 hours of each other even though the eggs in one nest were laid perhaps as much as two weeks apart.
As it seems, when the first young are ready to hatch they start a call resembling a pop-bottle rocket, even while still inside the egg, thus the hatching time is coordinated.
Greater Rheas are half-grown about three months after hatching, and sexually mature by their 14th month.
The Greater Rhea is considered a Near Threatened species according to the IUCN. The species is believed to be declining but it is still reasonably plentiful across its wide range, which is about 6,540,000 km2 (2,530,000 sq mi). The major factors in its decline are ranching and farming.
Farmers sometimes consider them pests, because they will eat broad-leaved crop plants, such as cabbage, chard and bok choi, although if very hungry, soybean leaves will do. Rhea’s disdain grasses unless there are no other options. Where they occur as pests, farmers tend to hunt and kill Greater Rheas.
This, along with egg-gathering and habitat loss, has led to the population decline.
The habitual burning of crops in South America has also contributed to their decline. Moreover, the birds’ health is affected by wholesale pesticide and herbicide spraying; while not threatening on a large scale, locally the species may be seriously affected by poisoning.
International trade in wild-caught Greater Rheas is restricted as per CITES Appendix II. The populations of Argentina and Uruguay are most seriously affected by the decline, in the former country mostly due to the adverse impact of agriculture, in the latter mostly due to overhunting in the late 20th century.
This species is farmed in North America and Europe similar to the Emu and Ostrich. The main products are meat and eggs, but rhea oil is used for cosmetics and soaps and rhea leather is also traded in quantity. Male Greater Rheas are very territorial during the breeding season. The infant chicks have high mortality in typical confinement farming situations, but under optimum free-range conditions chicks will reach adult size by their fifth month.
The Greater Rhea, Rhea americana derives its name from Rhea, a Roman god, and the Latin form of America. It was originally described by Carolus Linnaeus in his 18th-century work, Systema Naturae under its current binomial name.
He identified specimens from Sergipe, and Rio Grande do Norte, Brazil, in 1758. They are from the family Rheidae, and the Order Struthioniformes, commonly known as Ratites.